A smurf (in Counter Strike: Global Offensive) is any player who creates an alternate Steam account for the purpose of playing competitive matches against players several ranks lower than their own skill rank.

These players use this tactic often because of their inability to be successful at their own given rank, so they must 'Smurf' in order to feel better about themselves.

Another popular reason for 'Smurfing' is to give said player the illusion that they are actually close to being good enough to play at the professional level. Although, we all know they will never really amount to anything.

"This guy is dropping more than 40 kills in competitive against Silvers, he's a Smurf."
"I went 3-22 in my competitive match earlier today, so I created a Smurf account. I can't wait to destroy all those Silvers. I'm such a pro."

In advertising sales, a potential customer who talks about how big he is as a company and how he has all this advertising budget to spend with you but then he gets sticker shock when you present him a package at 100 per month.

Paul: Hey Mike did you sell that contractor you saw in the hamptons today?